1. Field of the Invention
The present invention generally relates to fuses included within semiconductor structures. More particularly, the present invention provides an electrical fuse that can be reprogrammed using a reverse electro-migration effect.
2. Related Art
As is known in the art, many modern semiconductor integrated circuits include fuses to protect sensitive parts during the manufacturing process, and for the activation of redundant circuits, such as redundant memory cells in the case of Dynamic Random Access Memories (DRAMs). There are typically two types of fuses, a laser-blowable fuse, and an electrically (e.g., current) blowable-fuse. Electrically blowable fuses provide an advantage over laser-blowable fuses in terms of size.
With laser blowable fuses, the fuses are typically formed at or near the surface of the integrated circuit. A laser beam striking the fuse material renders the fuse non-conductive, thereby inhibiting current from flowing through the fuse. Although laser blowable fuses are relatively simple to fabricate, there are disadvantages associated with them. For example, laser blowable fuses tend to be surface oriented, which places a limitation on the design of the integrated circuit. Further, laser blowable fuses tend to occupy a large amount of space on the surface of an integrated circuit, since adjacent fuses or devices must not be placed too close to the fuse or risk being inadvertently damaged by the laser beam during the fuse blowing operation.
Electrically blowable fuses, on the other hand, do not have to be placed at or near the surface of the integrated circuit. Accordingly, they give designers greater latitude in fuse placement. In general, electrically blowable fuses tend to be smaller than laser blowable fuses, which render them highly suitable for use in modern high density integrated circuits. Further, electrically blowable fuses have a high programming speed compared to conventional laser blowable fuses.
Various means have been used in the past to blow electrically blowable fuses. One recently used technique for opening the connection at the fuse employs the electro-migration effect, which has long been identified as a major metal failure mechanism. Electro-migration is the process whereby the ions of a metal conductor move in response to the passage of a high density current flow though the conductor. Such motion can lead to the formation of “voids” in the conductor, which can grow to a size where the conductor is unable to pass current. One can take advantage of the electro-migration effect to selectively open up metal connections (e.g., fuses) at desired locations within an integrated circuit.
One limitation of such electrically blowable fuses is they can be programmed only once (e.g., from a state “1” (conducting) to a state “0” (non-conducting)). In other words, once an electrically blowable fuse has been opened using the electro-migration effect it can not be closed again. Therefore, to reprogram or reconfigure an integrated circuit, redundant electrically blowable fuses and complicated supporting circuitry would be necessary.
Studies have been made regarding the healing of electro-migration related damage using a current reversal method. Evidence of such healing has been reported by E. Castano, et al, in a paper entitled “In Situ Observation of DC and AC Electro-migration in Passivated Al Lines,” Applied Physics Letters, Volume 59, Issue 1, Jul. 1, 1991, pp. 129-131. In this paper, it was shown that void size could be decreased by applying current stress in a reverse direction. As depicted in FIG. 1, for example, it was found that the average void size was reduced from 5.0 μm2 (point A) to 1.5 μm2 (point B) in less than one hour. A similar study was presented by J. Tao, et al. in a paper entitled, “An Electro-migration Failure Model for Interconnects under Pulsed and Bi-directional Current Stressing,” IEEE Trans on Electron Devices, Vol. 41, No. 4, April 1994, pp. 539-545. In this paper, it was shown that the resistance of a conductor made of Al/Si could be altered back and forth during forward and reverse current stressing as shown in FIG. 2. These and other such studies, however, have not provided a solution to the “programming only” nature of electrically blowable fuses that are programmed using the phenomenon of electro-migration.